1. Field of the Invention
The present invention relates to synthetic quartz glass for optics having a uniform transmittance and refractive index to radiation in the vacuum ultraviolet region from ArF and F2 excimer lasers. The invention relates also to a process for producing such synthetic quartz glass and a synthetic quartz glass substrate for use as photomasks.
2. Prior Art
Lithography systems plays the main role in the LSI manufacturing process, and one typical material used therein is quartz glass of high purity having high UV transmittance. Synthetic quartz glass is employed in lithography systems as stepper lens and photomask or reticle substrates which are used in the exposure and transfer steps of circuit patterns to silicon wafers.
The stepper apparatus generally includes an illumination section, a projection lens section and a wafer drive section. The illumination section converts light emitted by a light source into light of uniform intensity and guides it onto a photomask. The projection lens section plays the role of focusing the circuit pattern of the photomask onto a wafer in an accurate and reduced fashion. The materials of such components are essentially required to be highly transmissive to light from the light source.
As LSI chips continue to become more versatile and higher performing, research and development is actively underway to increase the level of device integration on wafers. Achieving higher device integration requires a high optical resolution capable of transferring very fine patterns. The resolution is represented by equation (1).R=k1×λ/NA  (1)                R: resolution        k1: coefficient        λ: wavelength of the light source        NA: numerical aperture        
Equation (1) suggests that there are two ways for achieving a high resolution. One way is to increase the numerical aperture. Increasing the numerical aperture, however, entails a reduction of focal depth. The currently used numerical aperture is thus thought to be almost the limit. The other way is to shorten the wavelength of the light source. Today, the predominant ultraviolet radiation utilized as the light source has a wavelength of 248 nm (KrF excimer laser). Intensive efforts are being made to move on to shorter wavelength 193 nm (ArF excimer laser), and further reduction to wavelength 157 nm (F2 excimer laser) is considered promising for the not-too-distant future.
Even for quartz glass having high UV transmittance, its transmittance gradually decreases in the vacuum ultraviolet region below 200 nm, and ceases altogether near 140 nm which is the absorption band attributable to the inherent structure of quartz glass.
Aside from quartz glass, fluoride single crystal is a candidate material for use in the vacuum ultraviolet region below 200 nm in wavelength, if transmittance is the only consideration. However, many problems including material strength, a coefficient of thermal expansion, and surface polishing necessary to use as lenses and photomask substrates must be overcome before the fluoride single crystal can be used on the practical level. Therefore, synthetic quartz glass is expected to play the very important role as the stepper component material in the future.
The transmittance by quartz glass in the range to the inherent absorption region is determined by the type and concentration of defect structures in quart glass. With respect to the F2 excimer laser having a light source wavelength of 157 nm, defect structures which affect transmittance include primarily Si—Si bonds and Si—OH bonds. Si—Si bonds, sometimes referred to as “oxygen deficiency defects,” have the central wavelength of absorption at 163 nm. Because these oxygen deficiency defects are also precursors of Si. defect structures (known as E′ centers) which have an absorption band at 215 nm, they cause serious problems not only when F2 (157 nm) is used as the light source, but also on use of KrF (248 nm) or ArF (193 nm). Si—OH bonds exhibit an absorption band near 160 nm. Therefore, the formation of defect structures must be minimized in order to produce quartz glass having a high transmittance in the vacuum UV region.
In the course of earlier research aimed at solving the above problem, quartz glass was produced by flame hydrolyzing a silica-forming reactant gas to form a porous silica matrix, then melting and vitrifying the porous silica matrix in a fluorine compound gas atmosphere. This method is successful in eliminating Si—OH bonds and instead, creating Si—F bonds in quartz glass. Si—F bonds have no absorption band above 140 nm since they have a larger band gap than Si—O bonds in quartz glass. Moreover, because Si—F bonds have a large bond energy and are very resistant to ultraviolet radiation, they do not form paramagnetic defects such as E′ centers when exposed to excimer laser irradiation.
Accordingly, to obtain a quartz glass well-suited to use as an optical material for vacuum ultraviolet-related applications, it is effective to create a high concentration of Si—F bonds within quartz glass. The resulting fluorine-doped quartz glass exhibits a very high transmittance to vacuum ultraviolet radiation (157 nm) of a F2 excimer laser.
Although the prior art method can create a high concentration of Si—F bonds in quartz glass, the resulting glass ingot has a substantially graded concentration between the interior and the periphery. As a consequence, there exists a transmittance distribution that the transmittance by quartz glass differs among positions at which vacuum UV radiation is irradiated. This problem was not fully overcome by the prior art method. It was thus difficult to produce quartz glass having a uniform transmittance distribution.
If quartz glass having uneven transmittance within it is used as a photomask substrate material, an image to be transferred becomes partially dim. Use of such photomask substrate is unacceptable. The uneven fluorine concentration causes not only an uneven transmittance, but also an uneven refractive index. The increased refractive index distribution within the substrate likewise inhibits an image from accurate transfer.
For the above-described reason, there is a strong desire to have a photomask-forming quartz glass substrate which is useful as an optical material for vacuum UV radiation and has a high transmittance, uniform distributions of transmittance and refractive index, and a low birefringence.